The Ultimate Guide to Understanding Engine Oil Viscosity

A package of automotive engine oil bears combinations of letters and figures and you are thinking what the heck is SAE 5W-20 or 5W-30? You want to find out what’s that mean. Some people found it’s too technical or too hard to understand. Know what? You have come to the right place. This article will help you see how easily you can understand engine oil viscosity.

What is Viscosity?

Viscosity is a measure of the resistance to flow of liquids. We all know that water, gasoline, and kerosene flow very readily. These liquids have low viscosities and are said to be “thin” or “light” liquids; on the other hand honey, molasses and asphalt flow very slowly and are said to have high viscosity or to be “thick” or “heavy”.

Unit of Viscosity

In order to measure viscosity, we must have units. Imagine a liquid to consist of several layers, and one layer moves relative to another layer at a definite velocity.

The resistance to the motion is a measure of the absolute viscosity: and if the layers are one metre apart and the velocity of movement one metre per second the viscosity is one Pascal second, which is the unit of absolute viscosity.

figure explain absolute viscosity

It is rather a large unit and normally we use one thousandth, or a millipascal second. This is numerically equivalent to an old unit know as the centipoise, which is still very often used.

Absolute viscosity is not conveniently measured with high accuracy and simpler methods of measurement of viscosity depending on flow through orifices or capillary tubes have been evolved.

The time of flow here is dependent not only the absolute viscosity, but on the pressure head driving the liquid through the orifice tube.

For a given height of liquid this is proportional to the density of the liquid. So we have another measurement of viscosity which depends on both the density and absolute viscosity and we call this the kinematic viscosity.

The normal unit for this is the centistoke, or square millimetre per second the only approved abbreviation for this is centistoke (cSt).

It is determined in specially designed viscometers based on bulbs and capillary tubes, with arrangements for a reproducible head of liquid during measurement.

Figure of Viscometer

The time taken to pass under this head of liquid from one timing mark to another is proportional to the kinematic viscosity. This is converted to absolute, sometimes called dynamic, viscosity by multiplying by the density at the same temperature.

There are other ways of measuring viscosity based on time of flow through an orifice. These are expressed as seconds, which represent the time of flow through an orifice. These are expressed as seconds, which represent the time of flow of a know volume in a particular apparatus.

The U.S. industry used the Saybolt Universal Seconds (S.U.S.), whereas in Britain the Redwood instrument, measuring Redwood No, 1 secons, was used. These units die hard, but centistokes are usually used except for some applications.

Since oils always become thinner as they are heated and increase in viscosity as they are cooled, the measured viscosity is dependent on temperature. Therefore, the temperature must always be stated when quoting viscosity.

For kinematic viscosity the normal temperatures for lubricants are 40C (104F) and 100C (212F), but you often find temperatures of 20, 50, 60, 70 and 80C (68, 122, 140,158, and 176F) specified , or sub-zero temperatures for fuels. Saybolt seconds were normally expressed at 100F and 210F, Redwood at 70F and 140F.

Viscosity Significance

Why it is important to ensure that a petroleum product has a correct viscosity. Where lubricants are concerned, the main function is to eliminate metal to metal contact by interposing a film of lubricant between moving surfaces.

Optimum Viscosity 

If the oil is too thin, it will tend to run off the surface and leave them dry, or leak through seals or past piston rings, if it is too thick, there will be a viscous “drag” between surfaces and machine efficiency will be reduced.

Each lubricant application has its optimum viscosity; and oil of that viscosity is specified. Generally, the thinnest possible oil which will stay on the surfaces without leakage and not be excessively consumed is best.

The viscosity depends on the optimum figure for the application, the commercial authority specification if it exists, the legal requirements associated with conforming to branded or advertised viscosity, and conformity to uniform viscosity specification.

Principal Authorities

There are two principal authorities which classify lubricant viscosity throughout the world; the Society of Automotive Engineers (SAE), based in USA and the International Organization for Standardization (ISO).

The SAE has traditionally classified viscosities for engine and gear lubricants whereas industrial lubricants have been covered by the ISO-VG (Viscosity Grade) System.


It should be made clear that these authorities do not stipulate the viscosity which should be used in an engine or machine – this is up to the equipment builder or the user.

It should be made clear that these authorities do not stipulate the viscosity which should be used in an engine or machine – this is up to the equipment builder or the user.

However, they do nominate numberical limits which define a viscosity grade specified by the equipment builder. Moreover they define a product by viscosity alone, and conformity which an SAE or ISO VG limit has no relation to quality.

Viscosity Index

The viscosity of oils is affected by their temperature, and since viscosity is an important property of any lubricant, we need to study these changes in greater details.

The effect of temperature changed is not uniform.

For example, any oil will change its viscosity much more between 10C (50F) and 15C (59F) than it will between 80C (176F) and 85C (185F).

In the practical design of lubrication systems they are often confronted with the necessity of finding out what the viscosity of an oil will be at, say 60C (140F), when its viscosity at 40C (104F) and 100C (212F) are known.

Because the viscosity change is non-uniform, this is a problem. Of course, one way of solving it is to send a sample of the oil to a laboratory, and to have its viscosity at 60C (140F) (or any other desired temperature) determined by direct measurement.

This is seldom possible, but fortunately there is an alternative.

It involves the use of special graph paper with non-linear scales constructed in such a manner that the viscosity-temperature relations for most hydrocarbons will plot as straight lines.

This paper is published by the ASTM. Using it, you can determine the viscosity of a hydrocarbon at any temperature if the viscosities at two other temperatures are known.

All oils do not behave in the same way

It was discovered that all oils do not behave in the same way as far as their temperature/viscosity relationships are concerned.

For example, suppose we have two oils which we shall call A and B. When their viscosities are measured at 100C (212F), both are found to be 20 cSt. This far, no problem.

We now determine their viscosities at 40C (104F), and we find that st this temperature the viscosity of A is 240 cSt, whereas B is 450 cSt. Evidently there is some fundamental difference between the two, B being much more affected by temperature than A.

The thing that is different is a property called Viscosity Index, usually abbreviated to VI.

VI is not a fundamental property of matter. It is a completely arbitrary scale, designed specifically for the needs of the oil industry.

The original concept was originated in 1929 by two American investigators names Dean and Davis. The lube stocks that had the least change in viscosity were assigned a VI of 100. The greatest change, on the other hand, these were assigned a VI of 0.

A high value of VI means a small change in viscosity and a low VI figure means a large change.

They had no way of knowing that the future would produce oils with VI values well in excess of 100, their system as originated could not accommodate this type of product, and had to be modified.

Viscosity Index Over 100

Many products on the market today have VI values well over 100. It is possible to produce this in straight lube stocks by improved refining methods.

Another source is synthesized hydrocarbons whose VI is 140 or more. But by far the commonest route to high VI is the use of an additive called a “VI improver”.

Calculations of VI by the Dean and Davis method lead to anomalous and contradictory results for high VI products and so a different method is used. You can find out more in ASTM D-2270 for fuller details.

Significance of Viscosity Index


"It is widely believed in the industry that a higher VI oil is “better”. While this in some circumstances is correct, it cannot by any means be accepted as a general truth."

Let’s first consider the accuracy with which VI can be determined. Since it is calculated directly from viscosity figures, any error in these will be reflected in the VI.

Furthermore, a very small error in viscosity determination can make a quite significant change in the VI, especially with low viscosity products.

Let’s consider an example using an oil of SAE 20 viscosity: -


Viscosity 40C (104F)

Viscosity 100C (212F)


Laboratory A

31.0 cSt

5.2 cSt


Laboratory B

31.2 cSt

5.16 cSt


Here we have two sets of viscosity determinations, which agree within industry standards for reproducibility and yet which give VI numbers differing by four. Any greater variation in viscosity determination will obviously lead to even larger discrepancies.

With higher viscosity materials, the precision improves but even so, a difference of less than five numbers is unlikely to be significant.

What VI tell us about an oil

Having gained an insight into the accuracy with which VI can be determined, we need to consider its significance. Listed below are some of the things that VI tells us about an oil.


Click to reveal more...

A higher VI oil changes less in viscosity with temperature.

The VI will give some indication of the type of hydrocarbons in the oil.

VI as a check on processing condition

In lube refining, VI is used as a check on processing condition, not so much because VI itself is important.

This is because it is an easily determined property, and has been found to correlate well with other properties, such as oxidation resistance, when everything else is equal.

This has led to the widespread belief, mentioned above, that the higher the VI, the ‘better” the oil.

The Higher The VI, The ‘Better” The Oil?

This has some validity if the two oils being compared are manufactured from the same crude and by the same refining process, with similar process conditions, and the difference in VI is significant.

When, however, the comparison is between two branded products, we will in all probability have different crudes and processing methods, and any comparison becomes meaningless unless the difference is very large indeed.

The SAE System


Everybody should be aware that a package of automotive engine oil bears combinations of letters and figures such as “SAE 30” or “SAE 20W-50”, etc. These tell us something about the contents of the package and in particular they convey some information about when and where they should be used.

The “SAE” is the Society of Automotive Engineers – a body which, among its activities, publishes standards for automotive components and materials as decided upon by its members.

One of these standards defines viscosities for engine and gear lubricants. Hence the first thing that needs to be understood is that SAE numbers refer only to viscosity and do not imply any other properties.

Image of engine oil with SAE viscosity grades

The first SAE lubricant classifications were published in 1911, their purpose being to provide automotive manufacturers and users with a common language that would ensure the use of a lubricant which was, at least, appropriate in viscosity.

Lubricants were classified in terms of Saybolt viscosities at 210F. This temperature was selected, firstly because it approximated to actual crank case temperatures which could be expected in summer, and secondly because it was a standard reference temperature in the industry.

The classifications were the wellknown 20, 30, 40, 50 and 60 grades which are still in use.  An SAE 20 oil was defined as one whose viscosity was between 45 and 58 SUS at 210F, SAE 30 was between 58 and 70, and so on.

It is interesting to note that in 1981 these classifications are still essentially the same, although now expressed in centistrokes and at 100C (212F). Most of the changes to the system have affected the method of describing low-temperature performance properties.

Low Temperature Properties

In 1911, most motorists laid up their cars for the winter months, and low temperature lubricant properties were therefore not regarded as important. But things changed. Year-round motoring became more common, as did electric starters.

image of 1917 motorists

These developments focused attention in low temperature behavior of the lubricating oil, and in 1923 the SAE added pour point requirements, which at least ensured that the lubricant would be fluid at the specified temperatures.

Ten years later, in 1933, a specification laying down actual viscosity limits, was added. The viscosities were specified, still in Saybolt seconds, at 0F, so the figures were arrived at by extrapolation, on an ASTM chart, from measured viscosities ar 100F and 210F.

Now it is not an easy matter to measure viscosities at 100F and 210F. Two classifications were at first introduced, 10W and 20W. The 20W classification was choosen so that a lubricant of 90-100 VI would meet the SAE 20 limits as well, and so it would be labeled SAE 20W-20.

In 1950 a 5W classification was added. The suffix “W” implied “winter” grade.

During the fifties, multigrade oils containing VI improver additives began to penetrate the market, and in the U.S. there was also a major swing to eight cylinder engines in passenger cars.

This combination began to produce a crop of cold weather starting difficulties, firstly because the bigger engines were harder to crank and, secondly because VI-improved oils are more or less non-Newtonian, especially at low temperatures, and hence extrapolated viscosities are not a reliable guide to low temperature performance.

Thus in 1967, extrapolated viscosities were replaced by actual viscosities (in poise) measured in a device known as a Cold Cranking Simulator, originally at 0F and more recently at -18C.

Figure of Cold Cranking Simulator

The Cold Cranking Simulator is a relatively simple device, consisting of a cyclindrical motor with two flats, revolving in a cylindrical housing through which chilled liquid can be circulated. The above diagram will make the details clear. The driving motor current is what is measures, and the apparatus needs to be calibrated using oils of know viscosity.


During the mid-seventies, there was pressure from European manufacturers to introduce a 15W classification. To undersand the need for this, one must digress a little to study the nature of multigrade oils.

A multigrade consists of a base stock plus a VI improver additive. As we have seen, it is required to comply with certain viscosity limits at -18C / 0F (as measured by CCS) and at 100C / 212F (as measured by kinematic viscometer).

As a broad generality, it may be said that the low temperature properties of a multigrade are determined by the base oil used in the blend, whereas the high temperature properties depend on the nature and quantity of the VI improver additive.

Hence a 10W-X blend will contain a lower viscosity base oil than a 20W-X.

Now viscosity measurement at 100C in the laboratory will give definite and repeatable results, but these measurements are made at very low shear rates, whereas in actual service in an engine, the lubricant is subjected to very high shear rates.

Given the non-Newtonian nature of VI-improved oils, some observers believe that laboratory measurements of viscosity do not necessarily correlate with effective wear-preventing viscosity as seen by the engine.

In short, the European manufacturers find that a 10W-X blend was, to put it as simply as possible, too thin to provide adequate protection for their small high-output engines operating at high speed on motorways.

Across the Atlantic, the American motorist’s big V-8 operating at legally imposed moderate speeds, had no such problem.

The obvious solution for Europe was therefore a 20W-X blend, but this gave rise to starting problems, because, again to put it simply, it is too thick at low temperature.

The compromise solution is the 15W classification which was introduced in 1977. This is not a separate classification. It simply identifies that the oil is an SAE 20W, but at the low end of the range.

As a genral rule, lube oil blenders will keep their low temperature viscosities close to the high end of therange, because lowering this figure will require more VI improver in the blend, and VI Improvers are expensive additives.

Summarizing then, an SAE 20W oil is one whose CCS viscosity at -18C (0F) is between 25 and 100 poises. To quote the SAE’s own words – “SAE 15W may be used to identify SAE 20W oils which have a maximum viscosity at -18C (0F) of 50 poises.

Later Developments

Even though the CCS viscosity defines one aspect of low-temperature performance, manufacturers found that they occasionally had engine failures leading to warranty claims due to engines being starved of oil at cold start-up conditions. Hence, there arose a need to measure “pumpability” as well as viscosity.

A further test has now been devised, using an instrument known as a “mini-rotary viscometer” (MRV). Without going into details, this instrument somewhat resembles a Brookfield, utilizing a revolving cylinder.

At the same time, the SAE has proposed some additional classification, and the range is now as set out below.

SAE Engine Oil Viscosity Classification: SAE J300

Table of Engine Oil Classification SAE J300


For motor oils “W” (0W, 5W, 10W, 15W, 20W, 25W) refers to the viscosity at 0F (-18C) as determined by the cold cranking simulator.

The straight figure (16, 20,30, 40, 50,60), refers to the viscosity at 100C (212F)

What does 5W-30 mean?

The W stands for Winter and refers to low-temperature performance relating to engine cranking speed and oil pumpability.

Grade 5W, from the top half of the table, this oil will have a cranking viscosity maximum of 6,600 mPa.s even in a cold winter night if its temperature should drop to -30C (-22F) and a maximum pumping viscosity of 61,000 mPa.s at temperature -35C (-31F).

Grade 30, from the bottom half of the table, this oil will have a low-shear-rate of kinematic viscosity in the range 9.3-12.5 cSt at 100C (212F) and a high-shear-rate viscosity no less than 2.9 mPa.s in the high temperature (150C / 302F) and high pressure part of an engine.

What is the different between 5W30 and 5W20?

5W30 vs 5W20? Basically, the higher the number, the higher the viscosity and the thicker the oil.

An SAE 5W-XX engine oil can be used down to -35C (-31F). An SAE 0W-XX can be used at a lower temperature as thinner but an SAE 10W-XX at a higher temperature as thicker.

5W30 vs  5W20

For SAE 5W-20 vs SAE 5W-30, the difference is the high temperature (100C / 212F) and HTHS (High Temperature High Shear 150C / 302F viscosities. 5W-30 has higher viscosity than 5W-20. The higher the viscosity, the thicker the oil.

SAE XW-20 will provide better fuel economy or provide more horsepower than an SAE XW-30 oil as less viscous & thinner providing less friction.

However, less viscous & thinner oils may not provide hardware durability leading to increased engine wear.

Why are 5W30 and 5W20 are so common?

SAE 5W30 and SAE 5W20 are so common because they are very thin oils that provide the best fuel economy which the motor manufacturers and governments of USA, Japan & Europe want today. Save fuel consumption with fewer exhaust emissions.

What Motor Grade Oil should I use?

The SAE Viscosity Grade you should use for a new vehicle is as the manufacturer states as it is a combination of fuel economy claims and engine durability. An engine has to be specifically designed for the low viscosity engine oils.

Oils are becoming thinner but advanced chemistry is required to provide required wear protection as with older thicker oils.

The future is for ultra low friction engines such that the SAE has introduced an SAE XW-16 classification, maybe SAE XW-4, 8 & 12 also.

For further detailed information refer to SAE Viscosity Grades, SAE J300.

Congratulations! You have made it to the end of the ultimate guide to viscosity! We hope this article is useful and help explain things in a way that easy to understand.

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Gary is the Technical Author and retired in 2011 after almost 39 years in the oil industry. He has been involved with Oil Industry Associations, Engine and Equipment Builders, Automotive Manufacturers, Racing Teams and Customers. Specializing in Automotive Engine Oils. He has worked in and visited countries and markets all over the Globe. His knowledge and experience will help you choose the best synthetic oil.

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